Shared Fields and Prototypical Inheritance

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Shared Fields and Prototypical Inheritance

snowguy13

I'm understanding JavaScript objects better now, so next comes inheritance. I know that you can add a field shared by all instances of an Object by referring to its prototype:
[COLORCODE=javascript]
function Object() {}
Object.prototype.field = "blah";
[/COLORCODE]

However, what happens to this concept when inheritance is applied?
[COLORCODE=javascript]
function SubObject() {}
SubObject.prototype = new Object();
SubObject.prototype.constructor = SubObject;
SubObject.prototype.field = "sub blah";
[/COLORCODE]

When I refer to SubObject.prototype.field, is the field "field" from the superprototype edited, or is a new field shared only by SubObjects created?

toicontien

Because "field" has a string value, no instances of SubObject share the same actual value. They all get copies of the string literal. Now if we do:

function Parent() {}
Parent.prototype = {
    foo: {}
};

Every instance of Parent shares the same instance of the object referred to using the foo property:

var a = new Parent();
var b  = new Parent();

alert(a.foo === b.foo); // alerts true

Now:

function Child() {}
Child.prototype = new Parent();
Child.prototype.foo = {};

var a = new Parent();
var b = new Child();

alert(a.foo === b.foo); // alerts false

All instances of Parent share the same foo property. All instances of Child share the same foo property, but a direct instance of Parent (a) does not share the same foo property as a direct instance of Child (b).

toicontien

Going back to your original question:

SubObject.prototype.foo is a direct member of the SubObject.prototype object. It has not edited Object.prototype.foo.

snowguy13

Ah, that makes sense. Okay, thank you!